1. Field of the Invention
The present invention relates to an optical module with a monitor photo diode, and more specifically to an optical module which has built therein a semiconductor laser and a photo diode for monitoring optical output power of the semiconductor laser.
2. Description of the Related Art
An optical module used in long-distance communications through an optical fiber principally uses a semiconductor laser (hereinafter might be called an xe2x80x9cLDxe2x80x9d for short) as a light source. Since optical output power of an LD is apt to change under the influence of a temperature change under a use environment of an optical module and secular deterioration of the LD, the optical module often incorporates a photo diode (hereinafter might be called a xe2x80x9cPDxe2x80x9d for short) therein and has the function of monitoring the optical output power of the LD. In this case, a forward optical output of the LD is generally optically-coupled to the optical fiber so as to be used for optical communications, and a backward optical output thereof is received by the PD to thereby control a drive current of the LD so that the forward optical output power is held constant.
Several PD mounting methods have been proposed for the optical module having incorporated therein the PD for monitoring the optical output power of the LD in this way. As the most common simple method, a PD is fixed to a base (which might be called by a name such as a submount, a header or the like) for the PD by solder or the like, and thereafter the PD base is fixed and mounted to the rear of the LD by solder or the like. As references having described such a structure, there are known Laid-Open Patent Publications xe2x80x9cSemiconductor Laser Device (Japanese Published Unexamined Patent Application No. Hei 6(1994)-289258)xe2x80x9d, xe2x80x9cOptical Module (Japanese Published Unexamined Patent Application No. Hei 9(1997)-21929)xe2x80x9d, etc.
As a method free of the use of the PD base, there is known an example wherein a groove (channel) is formed in a substrate with an LD mounted thereon by etching or the like, backward light of the LD is introduced into and reflected by the groove, an incident plane of a PD is mounted on the same plane as a surface plane of the substrate with the LD mounted thereon, so as to be substantially orthogonal to the optical axis of the backward light, thereby receiving the reflected light. As a reference having described such a structure, there is known, for example, xe2x80x9cOptical Subassembly Low in Height (Japanese Published Unexamined Patent Application No. Hei 9(1997)-222540)xe2x80x9d.
An advantage is brought about in that when a base for a PD is used, the PD can be mounted so that backward light of an LD is directly launched on the PD, relative to the spread of the backward light of the LD, whereby optical output power of the backward light can be detected with efficiency and the position to mount the PD can be determined freely in some measure. However, since the PD base is used, the number of parts is increased by one and hence the cost of each part increases. In addition, the number of connecting steps by solder or the like, and the number of electrical connections by wire bonding or the like are also respectively increased once. In the case of the solder-based connection in particular, temperature hierarchy of the solder is required to avoid re-melting of the previously-applied solder. Further, the degree of freedom of design on the solder""s temperature hierarchy is reduced over the whole optical module.
Further, such a structure that the backward light is introduced into and reflected by the groove of the LD mounting substrate without using the base for the PD to thereby launch it into the PD, is of a junction down LD (whose light emitting point is located on the side of an LD mounting substrate, which is close to a solder connecting plane) and is suitable for use in the case where the distance between a light emitting point of the LD and a light introducing groove is short. In the case of a junction up LD (whose light emitting point is located on the closer side of a surface plane of an LD mounting substrate, which is located on the side opposite to a solder connecting plane), however, the distance from a light emitting point of the LD to a light introducing groove increases and the amount of light introducible into a monitor decreases. Therefore, the amount of light received by the PD might lack to monitor the optical output power of the LD.
One advantage of the present embodiment is providing an optical module with a monitor photo diode, which is capable of reducing the number of parts constituting the optical module and increasing the amount of light introducible into the monitor PD form a light emitting point of an LD without using a base for the PD.
Another advantage of the present embodiment is to provide an optical module with a monitor photo diode which is capable of achieving the advantage referred to above and is applicable even to a junction up LD.
In order to achieve the above advantages, the present embodiment provides an optical module having built therein a semiconductor laser and a photo diode for monitoring optical output power of light emitted from the semiconductor laser, wherein the photo diode is directed fixed to a substrate equipped with the semiconductor laser mounted thereon, and a light receiving plane of the photo diode is formed at a predetermined angle (which ranges from 0xc2x0 to 60xc2x0) to the center of an optical axis of backward light of the semiconductor laser.
Now the substrate equipped with the semiconductor laser mounted thereon includes a case in which a semiconductor laser element is directly fixedly secured to a surface plane of the substrate, and a case in which it is fixedly secured to the surface plane of the substrate with a submount interposed therebetween. Either one of junction up and junction down may be used for the fixing of the photo diode and the semiconductor laser to the substrate. Further, the direct fixing includes a case in which the photo diode is soldered.
As preferred embodiments each provided with the predetermined angle, there are provided one wherein the photo diode is directly fixed to the substrate in such a manner that the light receiving plane of the photo diode is substantially parallel to the center of the optical axis of the backward light of the semiconductor laser and placed in the position where the light receiving plane has a predetermined offset with respect to the center of the optical axis of the backward light (there are additionally provided one wherein backward light is directly launched on its corresponding tight receiving plane and one wherein backward light is launched on its corresponding light receiving plane through the use of reflection of a side face of a photo diode, a surface plane of a substrate and faces of a groove formed in the substrate), one wherein a groove is formed in the substrate and the thickness of solder used to directly fix the photo diode to a groove internal face is controlled, one using an inclined face for defining a groove, and one wherein the thickness of a photo diode element is inclined.
These and other objects, features and advantages of the present invention will become more apparent in view of the following detailed description of the preferred embodiments in conjunction with the accompanying drawings.